System for performing spectral analyses under computer control

Abstract

Measurements of physical attributes such as dielectric film thickness that are susceptible to spectral analysis are accomplished rapidly and accurately by a spectrophotometric system in which a programmed digital computer operating concurrently with the optical scanning means automatically performs the calibrating, normalizing and data reducing functions that otherwise must be carried out as time-consuming human, mechanical or analog electronic operations. The control over the optical data handling operations exercised by the computer eliminates the need for mechanically or electronically adjusting the optical apparatus to meet changing system conditions, whether periodic or aperiodic. Source light is transmitted through a rotating variable-wavelength interference filter which acts during one-half of its cycle to transmit light of varying wave-length through a fiber-optic reference path directly to the optical data acquisition apparatus, while acting in the next half-cycle to transmit light of such varying wavelength indirectly to said data acquisition apparatus through a measurement path. In the present example, where film thickness is the attribute being measured, the measurement path comprises a bifurcated fiber-optic bundle, one branch of which is used to carry the light of variable wavelength to the sample, and the other branch of which carries light reflected from the sample to the aforesaid data acquisition apparatus. A computer program enables light passed through the reference path in one half-cycle to calibrate the system for measuring optical transmission or reflectance in the next half-cycle. Reduction of relative reflectance data to absolute reflectance data (needed for the accurate determination of film thickness) is accomplished by additional computer programs whose algorithms are based upon the discovery that all graphs of absolute reflectance versus wavelength for film samples of a given material having different thicknesses are bounded by a common pair of wave envelopes.

What is claimed is: 
     
       1. A method of operating a spectrophotometric system under the control of a digital computer to determine an attribute of a given material sample which is subject to spectral analysis, said method comprising the steps of: a. conducting to said sample, during each of a series of nonadjacent time periods, a beam of variable wavelength monochromatic light furnished by a given source, the wavelength of said beam varying at a given rate through a specified range of wavelength values during each of said periods;   b. conducting to a light detector in said system the light which comes from said sample when it is impinged by said beam;   c. conducting directly to said light detector, during time periods intervening the periods specified in step a, the beam of monochromatic light furnished by said source, the wavelength of which varies at said given rate through said specified range during each of said intervening periods;   d. converting the output of said detector during each of the periods specified in steps a and c to a sequence of stored digital values representing the respective intensities of light detected at a series of regularly timed intervals throughout the respective one of said periods;   e. operating said computer to derive from the sequences of values stored during any pair of successive steps a and c a new sequence of numbers representing the calculated optical responses of said sample to incident light having the wavelength of said variable-wavelength beam at each of said timed intervals under a hypothetical condition where it is assumed that the incident light has uniform intensity for all wavelengths in said range and all system components have constant operating characteristics; and   f. operating said computer to determine from said derived sequence of numbers the attribute of said sample which is being measured.   
     
     
       2. A method of operating a spectrophotometric system under the control of a digital computer to determine the thickness of a film of given dielectric material, said method comprising the steps of: a. conducting to said film, during each of a series of nonadjacent time periods, a beam of variable-wavelength monochromatic light furnished by a given source, the wavelength of said beam varying at a given rate through a specified range of wavelength values during each of said periods;   b. conducting to a light detector in said system the light reflected from said film as it is illuminated by said beam;   c. conducting directly to said light detector, during time periods intervening the periods specified in step a, the beam of monochromatic light furnished by said source, the wavelength of which varies at said given rate through said specified range of values during each of said intervening periods;   d. operating said computer to convert the output of said detector during each of the periods specified in steps a and c to a sequence of stored digital values representing the respective intensities of light detected at a series of regularly timed intervals throughout the respective one of said periods;   e. operating said computer to derive from the sequences of digital values stored during any pair of successive steps a and c, as described above, a series of numbers each representing the relative reflectance of said film when illuminated by monochromatic light having the wavelength of said beam at a respective one of said timed intervals under a hypothetical condition where it is assumed that the incident light has uniform intensity for all wavelengths in said range and all system components have constant operating characteristics; and   f. operating said computer to determine from said relative reflectance values the thickness of the film being measured.   
     
     
       3. In a spectrophotometric system having data acquisition means for generating digital value-representing signals in response to sensed optical inputs and also having a digital computer provided with data storage means and data processing means which are responsive to the output of said data acquisition means, a method of operating said system to measure the thickness of a film sample of given dielectric material, comprising the steps of: a. conducting to said film sample, during each of a series of nonadjacent time periods, a beam of variable-wavelength monochromatic light furnished by a given source, the wavelength of said beam varying at a given rate through a specified range of wavelength values during each of said periods;   b. conducting to a light-sensitive input device in said data acquisition means the light which is reflected from said sample when it is impinged by said beam;   c. conducting directly to said light-sensitive device, without impinging said sample and during time periods intervening those specified in step a, the beam of variable-wavelength monochromatic light furnished by said source, the wavelength of which varies at said given rate through said specified range of values during each of said intervening periods;   d. operating said computer to receive and store in said data storage means, at regularly timed intervals in each of the time periods described in a and c, the digital values generated by said data acquisition means, thereby to store at least two sets of numbers, one set representing the respective intensities of light detected by said light-sensitive device at said timed intervals during a period when the variable-wavelength light beam is being conducted directly to said device, and the other set of numbers representing the respective intensities of light detected by said device at said timed intervals during a period when the variable-wavelength light beam is impinging said film sample;   e. storing in said data storage means a set of numbers representing the respective wavelengths of the light beam at the respective ones of said timed intervals throughout any of said periods;   f. operating said data processing means to derive from all of the stored number sets recited hereinabove a table of values representing the manner in which the relative reflectance of said film sample varies with respect to the wavelength of the incident light beam under a hypothetical condition where it is assumed that the incident light has uniform intensity for all wavelengths in said range and all system components have constant operating characteristics;   g. operating said data processing means to multiply the relative reflectance values in said table by a factor which will convert the curve of relative reflectance versus wavelength to a curve which is at least approximately tangent to predefined upper and lower wave envelopes that bound the curves of absolute reflectance versus wavelength for all films of the given material having thicknesses within a predetermined thickness range; and   h. operating said data processing means to compute from the multiplied reflectance values the thickness of said film sample.   
     
     
       4. A computer-controlled spectrophotometric system for measuring a property of a given material sample which is subject to spectral analysis, said system comprising: a. cyclically operable illuminating means for furnishing a beam of monochromatic light, the wavelength of which varies periodically at a given rate through a specified range of wavelength values;   b. a light detector;   c. first light guiding means for conducting said light beam through a first path directly to said light detector during nonadjacent ones of the periods in which the wavelength of said light beam undergoes variation through said range of values;   d. second light guiding means effective during periods intervening said nonadjacent periods for conducting said light beam through a second path having an initial portion that directs said beam to said sample and a final portion that conducts light from said sample to said detector;   e. signal converting and data transfer means responsive to the output of said detector and operating in a timed relationship with the cyclic operation of said illuminating means for producing sequences of digital value representations, each such sequence denoting the variation of detected light intensity with respect to the wavelengths of said beam at a series of regularly timed intervals within each of said periods; and   f. a digital computer having data storage means for storing the sequences of digital value representations produced by said means e during at least two successive periods when said detector is receiving light through said first path and said second path, respectively, and having data processing means to calculate from such stored data the optical responses which would have been obtained from said sample at said timed intervals if the conditions had been such that the intensity of the variable-wavelength incident light were uniform at all wavelengths in said range and the operating characteristics of the components of said system were constant, thereby providing a new set of corrected values from which the measurement of said sample property can be accurately determined.   
     
     
       5. A spectrophotometric system as set forth in claim 4 wherein said illuminating means a comprises the following elements: a1. a source of polychromatic light;   a2. first light conducting means for directing polychromatic light from said source to an exit point optically aligned with but spaced from the entrance to said first path in said first light guiding means;   a3. second light conducting means for directing polychromatic light from said source to an exit point optically aligned with but spaced from the entrance to said second path in said second light guiding means; and   a4. a rotating variable-wavelength interference filter positioned so that it rotates alternately past the entrance to said first path and the entrance to said second path, the wavelength of the monochromatic light transmitted by said filter to either path depending upon the angular position of said filter.   
     
     
       6. A spectrophotometric system as set forth in claim 5 wherein each of said light conducting means and light guiding means is a fiber-optic bundle. 
     
     
       7. A system as set forth in claim 6 wherein said second light guiding means (d) is bifurcated fiber-optic bundle, one branch of which constitutes said initial portion, and the other branch of which constitutes said final portion. 
     
     
       8. A spectrophotometric system as set forth in claim 4 which includes timing means under the control of said illuminating means for causing said signal converting and data transfer means to sample the output of said light detector at predetermined wavelength increments during each of the periodic variations of the monochromatic beam wavelength. 
     
     
       9. A spectrophotometric system as set forth in claim 4 wherein said computer is programmed to divide each of the values in the sequence that was received through said second path by the corresondingly positioned value in the sequence that was received through said first path. 
     
     
       10. A spectrophotometric system as set forth in claim 4 wherein said second light guiding means is arranged so that light passed through said initial portion thereof is reflected from said sample and passed through said final portion thereof to said detector. 
     
     
       11. A system as set forth in claim 10 wherein said computer is programmed to divide each of the values in the sequence that was received through said second path by the corresondingly positioned value in the sequence that was received through said first path, thereby to yield said sequence of corrected values representing the relative reflectance of said sample at the respective wavelengths assumed by said beam at said timed intervals. 
     
     
       12. A system as set forth in claim 11 wherein said computer is programmed to multiply each of the values in said relative reflectance sequence by a factor which will cause the multiplied values to define a curve that is tangent to at least one of a pair of predetermined wave envelope curves which bound all curves of absolute reflectance versus wavelength for the given material, thereby providing a set of absolute reflectance values for said respective wavelengths. 
     
     
       13. A system as set forth in claim 12 wherein said computer is programmed to calculate from said sequence of absolute reflectance values the thickness of said sample. 
     
     
       14. In a computer-controlled spectrophotometric system of the kind wherein a light detector is arranged to respond to light of periodically varying wavelength which is received alternately through a sample measurement path and a direct reference path, the combination comprising: a. a rotatable variable-wavelength filter having a light-transmitting portion with an angular span not exceeding 180° positioned so that it transmits light into the respective entrances of said measurement path and said reference path during different periods in its rotation, the wavelength of the light transmitted into either of said paths at any instant depending upon the angular position of said filter relative to the path entrance;   b. a polychromatic light source; and   c. means providing light conducting paths leading from said light source to exits on the side of said filter which is farthest from said measurement path and reference path entrances, said path exits being respectively aligned optically with said entrances, whereby the rotation of said filter causes polychromatic light emerging from said path exits to be transmitted by said filter as monochromatic light of varying wavelength to said measurement path and said reference path, respectively, during different periods in the rotation of said filter.   
     
     
       15. The combination set forth in claim 14 wherein said means c is a split fiber-optic bundle having branches leading respectively to said exits and having a common portion adjacent to said light source wherein the fibers of said branches have a randomized distribution. .Iadd. 16. A method of operating a spectrophotometric system under the control of a digital computer to determine an attribute of a given material sample which is subject to spectral analysis, said method comprising the steps of: a. conducting to said sample, during each of a series of nonadjacent time periods, a beam of light which includes at least part of the light furnished by a given polychromatic light source;   b. conducting to a light detector in said system a beam of light which comes from said sample when it is illuminated by the first-mentioned light beam during each of said nonadjacent periods;   c. conducting to said detector through a path that is independent of said sample, during time periods intervening the nonadjacent time periods in the first-mentioned series, a beam of light which includes at least part of the light furnished by said light source;   d. selectively filtering, in a time-varying fashion, the light which passes from said source to said detector by way of said sample during each of said first series of time periods and the light which passes from said source to said detector independently of said sample during each of said intervening periods for causing the light which is thereby furnished to said detector to be a monochromatic light beam whose wavelength varies as a given function of time through a specified range of wavelength values during each of said nonadjacent periods and each of said intervening periods;   e. converting the output of said detector during each of said nonadjacent and intervening periods to a sequence of stored digital values representing the respective intensities of light detected at a series of predefined time intervals throughout the respective one of said periods;   f. operating said computer to derive from the sequences of values stored during any pair of successive periods, including one of said first series of nonadjacent periods and one of said intervening periods, a new sequence of numbers respectively representing the calculated optical responses of said sample to light having the wavelength of said monochromatic light beam at said predefined time intervals under a hypothetical condition where it is assumed that the light conducted to said sample has uniform intensity for all wavelengths in said range and that all system components have constant operating characteristics; and   g. operating said computer to determine from said derived sequence of numbers the attribute of said sample which is being measured.   
     
     
        .Iaddend..Iadd. 17.  A method as set forth in claim 16 wherein the sample is a dielectric film, and the attribute being measured is the film thickness. .Iaddend..Iadd. 18. A method as set forth in claim 16 wherein the light conducted to said detector from said sample is light which has been reflected by said sample from said first light beam. .Iaddend. .Iadd. 19. A method as set forth in claim 18 wherein the sample is a dielectric film, and the attribute being measured is the film thickness. .Iaddend..Iadd. 20. A computer-controlled spectrophotometric system for measuring a property of a given material sample which is subject to spectral analysis, said system comprising: a. a source of polychromatic light;   b. a light detector;   c. first light guiding means for conducting light from said source through a first path to said light detector, said first path including an antecedent portion for directing light to said sample and a subsequent portion for directing light from said sample to said detector;   d. second light guiding means for conducting light from said source through a second path to said light detector, said second path being independent of said sample;   e. optical filtering means positioned in each of said light paths for converting polychromatic light to monochromatic light having a wavelength that varies as a given function of time through a predetermined range of wavelength values during each of a series of time periods, said filtering means being so arranged that monochromatic light of any particular wavelength in said range is directed to said detector through one of said paths at a time that is displaced by a predetermined amount from the time at which monochromatic light of said particular wavelength is directed to said detector through the other of said paths;   f. signal converting and data transfer means responsive to the output of said detector and operable in timed relationship with said filtering means for producing sequences of digital value representations, each such sequence denoting the variation of detected light intensity with respect to wavelength at a series of predefined time intervals within each of said periods; and   g. a digital computer for storing the sequences of digital value representations produced by said signal converting and data storage means as it receives variable-wavelength monochromatic light through said first path and said second path, respectively, and for calculating from such stored data the optical responses which would have been obtained from said sample if the conditions had been such that the intensity of the light conducted to said sample were uniform at all wavelengths in said range and the operating characteristics of the system components were constant, thereby providing a new set of corrected values from which the measurement   
     
     
        of said sample property can be accurately determined. .Iaddend..Iadd. 21. A system as set forth in claim 20 wherein said optical filtering means is a rotatable variable-wavelength filter so positioned that during its rotation it moves alternately across said first path and said second path, the wavelength of the monochromatic light which is transmitted by said filter to the succeeding portion of each path at any instant depending upon the angular position of said filter. .Iaddend. .Iadd. 22. A system as set forth in claim 20 wherein each light guiding means includes a fiber-optic bundle, said first light guiding means having a bifurcated fiber-optic bundle, one branch of which is in the antecedent portion of said first path, and the other branch of which is in the subsequent portion of said first path, whereby said detector receives light which is reflected from said sample. .Iaddend..Iadd. 23. A system as set forth in claim 20 wherein said first light guiding means is arranged so that light passed through said antecedent portion thereof is reflected from said sample and is then passed through said subsequent portion thereof to said detector. .Iaddend..Iadd. 24. A system as set forth in claim 23 wherein said computer is programmed to divide each of the values in the sequence that was received through said first path by the correspondingly positioned value in the sequence that was received through said second path, whereby said sequence of corrected values will represent the relative reflectance of said sample at the respective wavelengths assumed by said beam at said timed intervals. .Iaddend..Iadd. 25. A system as set forth in claim 24 wherein said computer is programmed to multiply each of the values in said relative reflectance sequence by a factor which will cause the multiplied values to define a curve that is tangent to at least one of a pair of predetermined wave envelope curves which bound all curves of absolute reflectance versus wavelength for the given material, thereby providing a set of absolute reflectance values for said respective wavelengths. .Iaddend. .Iadd. 26. A system as set forth in claim 25 wherein said computer is programmed to calculate from said sequence of absolute reflectance values the thickness of said sample. .Iaddend.